Freeing obsolete update lists can be slow during checkpoints

At the end of update_serial we check for obsolete update structures. If we can't evict b/c of a checkpoint and that list grows large, this is wasted work inside a transaction.

Updates shouldn't be blocked, that is, only a single thread at a time is doing the check for obsolete update structures.

It would be pretty easy to turn off the check for obsolete update structures when checkpoint is running on this file; let's explore ways to tighten it down further using the checkpoint generation and other transactional information.